The slk* functions manipulate the set of soft function-key labels that exist on many terminals. For those terminals that do not have soft labels, curses takes over the bottom line of stdscr, reducing the size of stdscr and the variable LINES. curses standardizes on eight labels of up to eight characters each. In addition to this, the ncurses implementation supports a mode where it simulates 12 labels of up to five characters each. This is useful for PC-like enduser devices. ncurses simulates this mode by taking over up to two lines at the bottom of the screen; it does not try to use any hardware support for this mode.

The slk_init routine must be called before initscr or newterm is called. If initscr eventually uses a line from stdscr to emulate the soft labels, then fmt determines how the labels are arranged on the screen:

0

indicates a 3-2-3 arrangement of the labels.

1

indicates a 4-4 arrangement

2

indicates the PC-like 4-4-4 mode.

3

is again the PC-like 4-4-4 mode, but in addition an index line is generated, helping the user to identify the key numbers easily.

The slk_attron, slk_attrset, slk_attroff and slk_attr routines correspond to attron, attrset, attroff and attr_get. They have an effect only if soft labels are simulated on the bottom line of the screen. The default highlight for soft keys is A_STANDOUT (as in System V curses, which does not document this fact).

returns an error if the terminal or the softkeys were not initialized, or the labnum parameter is outside the range of label counts, or if the format parameter is outside the range 0..2, or if memory for the labels cannot be allocated.

X/Open Curses documents the opts argument as reserved for future use, saying that it must be null. This implementation uses that parameter in ABI 6 for the functions which have a color-pair parameter to support extended color pairs.

For functions which modify the color, e.g., slk_attr_set, if opts is set it is treated as a pointer to int, and used to set the color pair instead of the short pair parameter.

The XSI Curses standard, Issue 4, described the soft-key functions, with some differences from SVr4 curses:

It added functions like the SVr4 attribute-manipulation functions slk_attron, slk_attroff, slk_attrset, but which use attr_t parameters (rather than chtype), along with a reserved opts parameter.

Two of these new functions (unlike the SVr4 functions) have no provision for color: slk_attr_on and slk_attr_off.

The third function (slk_attr_set) has a color-pair parameter.

It added const qualifiers to parameters (unnecessarily), and

It added slk_color.

The format codes 2 and 3 for slk_init and the function slk_attr are specific to ncurses.

X/Open Curses does not specify a limit for the number of colors and color pairs which a terminal can support. However, in its use of short for the parameters, it carries over SVr4's implementation detail for the compiled terminfo database, which uses signed 16-bit numbers. This implementation provides extended versions of those functions which use short parameters, allowing applications to use larger color- and pair-numbers.